Diffuse reflectance (DR) spectrometry is becoming an important technique for characterizing powdered samples; however, surprisingly little is known about the technique, and its utility will be limited until the quantitative behavior is more thoroughly understood. The effect of optical geometry on diffuse reflectance spectra of several types of samples including materials without sharp absorption bands, neat powdered materials (organic amterials with weak absorption bands and inorganic materials with bands of greater absorption), effect of particle size for all samples, and the effect of dilution of the samples in KCl will be studied as will varying the angle of incidence and the angle of reflection. Also to be studied is the feasibility of measuring the DR spectra of powders dispersed in liquids. Such measurements could have considerable significance in the detection of low levels of drugs and their metabolites in physiological fluids. In addition, because a number of manufacturers produce diffuse reflectance spectrometers which show differences which can (and do) affect the spectra, it is important to determine the optimum geometry for DR spectroscopy so that a consistency between commercial instruments can be obtained.